1. Field of the Invention
The present invention relates to a light emitting device, more particularly to a group III-nitride light emitting device having enhanced brightness and electrical properties.
2. Description of the Related Art
Since development of a group III-nitride light emitting diode (LED) using AlxGayIn1−x−yN-based material, a great deal of efforts have been made to heighten brightness. A method for increasing brightness includes enhancing light emitting efficiency of a light emitting layer and extracting light generated inside into the outside without internal loss.
Several technologies have been suggested to improve external extraction efficiency (external quantum efficiency) of light generated inside a device. For example, a Ni/Au layer having a relatively good transmissibility is thinly deposited with a thickness of up to 100 Å on a p-type clad layer to be used as a p-electrode of a group III-nitride light emitting device. This allows light generated inside to exit to the outside through the Ni/Au layer (light exits in the p-electrode side). However, even despite deposition of the Ni/Au layer with a small thickness, a considerable amount of light is absorbed in the Ni/Au layer or reflected into the inside again. Also, sapphire (Al2O3) used as a substrate has low thermal conductivity so that heat generated during operation of the device is hardly released to the outside, thus deteriorating device properties.
To overcome such problems and elevate external extraction efficiency more significantly, a group III-nitride light emitting device with flip chip structure has been developed. In this case, for a metal layer used for a p-electrode, a Ni/Al layer, Ni/Ag layer or Ni/Al(Ag)/Pt layer having good reflectivity is used instead of the Ni/Au layer having good transmissibility (Al or Ag has higher reflectivity than Au). U.S. Pat. No. 6,445,011 discloses a light emitting device with flip-chip structure connected to electrodes of a submount.
FIG. 1 is a sectional view illustrating an example of a conventional flip chip light emitting device. Referring to FIG. 1, the light emitting device 10 includes an undoped GaN layer 13, an n-doped AlGaN layer 15, an active layer 17 and a p-doped AlGaN layer 19 sequentially formed on a transparent sapphire substrate 11. A p-electrode 21 made of Ni/Ag (or Al)/Pt layer (Ni is placed below Pt) having high reflectivity is formed on a p-doped AlGaN layer 19. An n-electrode 23 is formed on an area of the n-doped AlGaN layer 15. The flip chip light emitting device 10 is flipped over so that the substrate 11 faces upward, and then mounted on the submount. FIG. 2 shows a side sectional view illustrating a flip chip light emitting device 10 mounted on the submmount.
Referring to FIG. 2, the flip chip light emitting device 10 is mounted on a submount 21 so that the p-electrode 21 and n-electrode 23 of the light emitting device 10 are connected to an electrode part (not illustrated) of the submount 21 through bumps 31, 33. In this flip chip structure, light exits toward the transparent sapphire substrate 11. The p-electrode 21 uses an Ag(or Al) layer having higher reflectivity to reflect light upward (toward the output direction) through the Ag(Al) layer. This enhances external extraction efficiency of light.
In the p-electrode 21 made of the Ni/Ag(Al)/Pt layer, thick Ni layer reduces reflectivity of the p-electrode 21, while too thin Ni layer causes p-electrode 21 to be opened. The opened p-electrode 21 leads to defects of the device. Therefore, the Ni layer used should have appropriate thickness, and the Ag (Al) layer should be used as a reflective film. But use of the Ni layer having even adequate thickness results in decreased brightness.